In order to improve device performance, semiconductor device circuit densities continue to increase. This increase in circuit density is realized by decreasing feature sizes. Current technologies target feature sizes of 0.15 microns and 0.13 microns with further decreases expected in the near future.
The exact dimensions of features within the devices are controlled by all steps in the fabrication process. Vertical dimensions are controlled by doping and layering processes where as horizontal dimensions are determined primarily by photolithographic processes. The horizontal widths of the lines and spaces that make up the circuit patterns are often referred to as critical dimensions (CD).
Photolithography is the technique used to form the precise circuit patterns on the substrate surface. These patterns are transferred into the wafer structure by a subsequent etch or deposition process. Ideally the photolithography step creates a pattern that exactly matches the design dimensions (correct CD) at the designed locations (known as alignment or registration).
Photolithography is a multi-step process where the desired pattern is first formed on a photomask (or reticle). The pattern is transferred to the substrate through a photomasking operation where radiation (e.g., UV light) is transmitted through the patterned photomask exposing a radiation sensitive coating on the substrate. This coating (photoresist) undergoes a chemical change upon exposure to the radiation rendering the exposed areas either more or less soluble to a subsequent development chemistry. Photolithography techniques are well known in the art. An overview of these techniques can be found in the text Introduction to Microlithography edited by Thompson et. al.
Since the photomask acts as the master for generating the circuit patterns on a large number of substrates, any imperfections introduced during the manufacturing of the photomask will be replicated on all wafers imaged with that photomask. Consequently, fabricating a high quality photomask that faithfully represents the designed patterns and dimensions is critical to creating high yield device manufacturing processes.
There are two major types of photomask reticles that are well known in the art: absorber and phase shifting. An absorber photomask typically consists of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with an opaque film (e.g., Cr). The opaque film may consist of a single layer or multiple materials (e.g., an anti-reflective layer (AR chromium) on top of an underlying chromium layer). In the case of binary chromium photomasks, examples of commonly used opaque films (listed by trade name) include, but are not limited to, AR8, NTAR7, NTAR5, TF11, TF21. During fabrication of the photomask, the opaque film is deposited on the transparent substrate. A photoresist layer is then deposited on top of the opaque layer and patterned (e.g., exposure to a laser or electron beam). Once exposed, the photoresist layer is then developed to expose areas of the underlying opaque film that are to be removed. A subsequent etch operation removes the exposed film forming the absorber photomask.
There are two subcategories of phase shifting masks that are well known in the art: alternating and embedded attenuating masks. Alternating phase shift masks typically consist of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with an opaque film (e.g., Cr and antireflective Cr). During fabrication of the photomask, the opaque film is deposited on the transparent substrate. A photoresist layer is then deposited on top of the opaque layer and patterned using a laser or electron beam. Once exposed, the photoresist layer is then developed to expose areas of the underlying opaque film that are to be removed. An etch process removes the exposed opaque film exposing the underlying substrate. A second process is used to etch a precise depth into the underlying substrate. Optionally the substrate may be subjected to a second photoresist coat and develop process prior to the second etch process as is known in the art.
Embedded attenuating phase shift masks (EAPSM) typically consist of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with a film or stack of films that attenuate the transmitted light while shifting the phase 180 degrees at a desired wavelength. An opaque film or film stack (e.g., Cr and antireflective Cr) is then deposited on the phase shift material. A photoresist layer is then deposited on top of the opaque layer and patterned (e.g., using a laser or electron beam). Once exposed, the photoresist layer is then developed to expose areas of the underlying opaque film that are to be removed. An etch process is then used to remove the exposed opaque film exposing the underlying phase shifting/attenuating film or film stack. Following the etching of the opaque film, a second etch process is used to etch the phase shift layer stopping on the underlying substrate. Alternatively, an etch stop layer may be present between the phase shift layer and the substrate, in which case the second etch process will selectively stop at the etch stop layer.
Ideally, the etch process will have a high etch selectivity to both the topmost etch resistant mask (e.g., photoresist, e-beam resist, etc.) and underlying material (substrate or etch stop) while creating features that have smooth vertical side walls that exactly replicate the CD of the original mask (e.g., photoresist) pattern. Wet etch processes (e.g., aqueous solutions of chloric acid and ceric ammonium nitrite for AR Cr/Cr etch) show good etch selectivity to the etch mask and underlying substrate, but are isotropic and result in significant undercut of the mask and result in sloped feature profiles. The undercut and sloped feature profiles result in changes to the etched feature CD. The undesirable change in CD and/or sloped feature profiles degrade the optical performance of the finished photomask.
Dry etch (plasma) processes are a well known alternative to wet etch processing. Plasma etching provides a more anisotropic etch result than wet processes. Dry etching is commonly used in the fabrication of all three mask types. In the case of binary Cr photomasks, a mixture of chlorine containing gas and oxygen containing gas are typically used. Additional gas components including inerts and passivants which have been used to improve process performance.
Early dry etch work on photomasks utilized low density (˜109 ion/cm3) plasma in a capacitively coupled (diode) reactor while most current dry etch photomask processes utilize high density (1010-1012 ions/cm3) configurations (e.g., inductively coupled plasma (ICP), transformer coupled plasma (TCP), electronic cyclotron resonance (ECR), etc.).
For the case of a dry etch process for a binary Cr photomask, the process typically consists of three primary steps. The first step removes the antireflective coating (e.g., chrome oxide, chromium nitride, chromium oxynitride) using a chlorine containing plasma (e.g., Cl2, HCl, CCl4, BCl3). Optionally, the AR Cr etch step may include an oxygen containing gas (e.g., O2, CO, CO2, N2O, NO2, SO2, etc.) as well as inert gases (e.g., He, Ar, Ne, Xe, Kr, etc.). The first step may be run on a timed basis, or terminated at the AR Cr/Cr interface through the use of an endpoint technique (e.g., laser reflectance spectroscopy, optical emission spectroscopy).
The second step etches the bulk Cr material stopping on the underlying film or substrate. The process gas mixture for the second step typically contains a chlorine source and an oxygen source. As with the first step, the process gas mixture may also contain inert gases. Furthermore, the first and second steps may have identical process conditions. Optionally, the second step may be terminated through the use of an endpoint technique.
The third step is an overetch step to ensure that areas of different Cr loadings are completely cleared. The over etch step is also used to improve the sloped profiles seen in low Cr density areas. While longer overetch times ensure that high density Cr areas are completely cleared with improved (more vertical) feature profiles, longer overetch times also result in more lateral etching and higher CD bias. The overetch step parameters may be identical to either (or both) of the first and second step recipes. The duration of the overetch step is typically based on a percentage of the duration of a preceding step.
Optionally a descum or trim step may be performed prior to etching the AR chrome layer in order to improve the etch mask (e.g., photoresist) profile.
While plasma etch processes are more anisotropic than wet etch processes, they can still introduce dimensional changes in the patterned material. The degree of loss or gain in CD introduced during the etch process is referred to as “CD bias.” CD bias for the etch process can be calculated by taking the final feature CD after the etch process and subtracting the initial CD of the feature before etch. It is desirable to minimize the extent of lateral etching that contributes to CD bias.
In the cases where the process CD bias is non-zero, the CD bias uniformity must be considered. The CD bias uniformity is the distribution of values around the average CD bias value. The CD bias uniformity may have both systematic and random components. One systematic non-uniformity that has been observed in photomask etching corresponds to local etch loading effects (e.g., micro-loading or loading effect).
The phenomena commonly referred to as “load dependence” is known in the art for dry etch processes. Load dependence refers to the relationship between the area of exposed material to be etched and the material etch rate. For example, in a binary Cr photomask dry etch process, the vertical Cr etch rate is lower in areas with higher Cr densities. Presuming that lateral etch rate is also load dependent, it is reasonable to expect that higher Cr densities will have lower lateral etch rates and as a result a lower CD bias. In practice however, the opposite is observed—higher Cr density features (with lower vertical etch rates) typically have a higher CD bias when compared to lower Cr density (lower load) areas.
In order to evaluate the CD performance of a process, two factors need to be considered: the lateral etch rate of the Cr film stack, and the final profile of the etched feature. In the case of Cr etching, where the pattern contains areas of different Cr loading, features in the higher Cr density (high photoresist density or clear) areas typically etch at slower etch rates (as expected) but show a larger CD bias as compared to low load areas (unexpected).
There is a need for an improved method to fabricate a photomask with improved feature profiles and CD performance.
Nothing in the prior art provides the benefits attendant with the present invention.
Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement of the semiconductor processing art.
Another object of the present invention is to provide a method for processing a photolithographic substrate, comprising loading the photolithographic substrate into a vacuum chamber; cooling the photolithographic substrate to a target temperature; introducing at least one processing gas into said vacuum chamber; igniting a plasma from said processing gas after said cooling step; processing the photolithographic substrate using said plasma; and unloading the photolithographic substrate from said vacuum chamber.
Yet another object of the present invention is to provide a method for processing a photolithographic substrate, comprising loading the photolithographic substrate onto a substrate support within a vacuum chamber; controlling the temperature of the photolithographic substrate through a fluid; introducing at least one processing gas into said vacuum chamber; igniting a plasma from said processing gas; processing the photolithographic substrate using said plasma; and unloading the photolithographic substrate from said vacuum chamber.
Still yet another object of the present invention is to provide a method of etching a photolithographic substrate, comprising loading the photolithographic substrate onto a substrate support within a vacuum chamber; introducing at least one processing gas into said vacuum chamber; igniting a first plasma from said processing gas; etching the photolithographic substrate for a first set of process conditions using said first plasma; cooling the photolithographic substrate to a target temperature; igniting a second plasma from said processing gas after said cooling step; etching the photolithographic substrate for a second set of process conditions using said second plasma; and unloading the photolithographic substrate from said vacuum chamber.
Another object of the present invention is to provide a method of controlling the temperature of a substrate with a high thermal mass during a plasma process, comprising adjusting the temperature of the substrate to a target temperature; loading the substrate onto a substrate support within a vacuum chamber; introducing at least one processing gas into said vacuum chamber; igniting a plasma from said processing gas; processing the substrate using said plasma; and unloading the substrate from said vacuum chamber.
Yet another object of the present invention is to provide a method of etching a photolithographic substrate, comprising loading the photolithographic substrate onto a substrate support within a vacuum chamber; introducing at least one processing gas into said vacuum chamber; igniting a plasma from said processing gas; etching the photolithographic substrate at a first target temperature at a first set of process conditions using said plasma; etching the photolithographic substrate at a second target temperature at a second set of process conditions using said plasma; and unloading the photolithographic substrate from said vacuum chamber.
The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.